Method for fabrication of optical element, and optical element having three-dimensional laminated structure

ABSTRACT

A method for production of an optical element includes preparing a first member having formed on the surface of a first substrate  101  a first layer  103  by at least one of epitaxial growth and pore-making (micropore-making) and a second member having a porous layer for layer separation formed on a second substrate  104  and having formed thereon a second layer by at least one of epitaxial growth and pore-making (micropore-making), bonding the first layer  103  and the second layer, separating the second substrate  104  and the second layer of the second member from each other at the porous layer for layer separation in the second member, to form a laminated structure on the first substrate  101 , forming a refraction index distribution pattern produced by a difference in refraction index in the plane of at least one of the first layer  103  and the second layer.

TECHNICAL FIELD

The present invention relates to an optical element having athree-dimensional laminated structure, concerned with electromagneticwaves including visible light, terahertz waves, microwaves and X-rays,and a method for fabrication of the optical element. Optical elementsare widely used to display and transfer images, in opticalcommunications, including data communications, information processorsusing light, and sensors and detection systems for detecting varioustypes of information, such as image and biological information, at ahigh level of sensitivity.

BACKGROUND ART

In recent years, an optical material called photonic crystals having aperiodic refractive index distribution and techniques using the materialhave been receiving attention. The techniques include, for example,techniques for fabricating photonic crystals with an optical material,and techniques utilizing behavior of light in photonic crystals andtechniques utilizing a phenomenon in which a luminous state of aluminous material existing in photonic crystals is controlled (E.Yablonovitch: “Phys. Rev. Lett” Vol. 58, p. 2059, 1987). A possibilityof applying these techniques to the optical element is controversial.

In association with the technique of the optical element, so called DFB(distributed feedback) lasers effectively using one-dimensional periodicstructures for semiconductor lasers and the like have gone into actualuse. Basic studies for applying two-dimensional photonic crystalsincluding a two-dimensional periodic arrangement of cylindrical pores tooptical communication parts are vigorously conducted.

In two-dimensional photonic crystals, however, performance of lightcontrol in one non-periodic direction (direction of thickness ingeneral) is inferior to performance of light control in other twoperiodic directions. This raises a problem when various opticalelements, including optical communication parts, and systems using lightare built. Some attempts are made to use three-dimensional (3D) photoniccrystals forming periodic structures in all three directions.

Examples of three-dimensional photonic crystals, which have beendeveloped to date, include crystals called lattice type crystals or woodpile type crystals (U.S. Pat. No. 5,335,240 (Ho et al.), Noda: “PhotonicCrystals—Application, Technology and Physics,” p. 128, 2002, CMCpress.), crystals fabricated by a micromechanic fabrication method(Hirayama et al: “Photonic Crystals—Application, Technology andPhysics—,” p. 157, 2002, CMC press.), and crystals fabricated by a thinfilm deposition method called self cloning (S. Kawakami “Fabrication ofsubmicrometre 3D periodic structures composed of Si/SiO₂,” Electron.Lett., Vol. 33, pp. 1260-1261 (1997), International Patent Laid-openWO98/44368 publication and Sato: “Photonic Crystals—Application,Technology and Physics,” p. 229, 2002, CMC press.).

The biggest challenge in attempts to fabricate such three-dimensionalphotonic crystals is fabrication of a complicated three-dimensionalstructure in a fine period. The challenge is a technique for fabricatinga three-dimensional form in which the period required in photoniccrystals intended for near-infrared light wavelengths, visible lightwavelengths, ultraviolet light wavelengths and the like, which areimportant especially in terms of application, is 1 μm or less,particularly on the order of 100 nm. Quality, such as dimensionalaccuracy and interface roughness, at 1 to 2 orders below the period isconsidered important. Values required as dimensional accuracy androughness of the surface and the side face are, for example, about 1 to10 nm. The roughness of the surface and the side face at this levelcauses scattering of light. Scattering of light causes a considerableloss in photonic crystals using multiple reflection and multiple beaminterference as an operational principle, resulting in significantdegradation in performance of the element.

For achieving a practical product, it is important that a plurality ofelements can be fabricated at a time from a wafer material having alarge area as in the case of many semiconductor parts. For example, ifelements that are generally 10×10 μm² to 1×1 mm² in size can befabricated from a wafer having an area of about 100×100 mm², the numberof elements that can be made from one wafer is increased and the cost isreduced.

It is very important that one element itself has a large area. This isbecause an element having a large area can provide a display or systemin a form of one element.

For such needs, it is difficult to fabricate three-dimensional photoniccrystals having a large area in a sufficiently high quantity and goodquality using a conventional method. Specifically, in a conventionalmethod in which the lattice of a compound semiconductor, such as GaAs,is welded, it is difficult to increase an area because the size of asubstrate, such as GaAs, is limited, and it is difficult to reduce thecost of fabricating three-dimensional photonic crystals required to havea plurality of layers because such a substrate is expensive. Even if amethod of depositing layers by micromechanic handling is used, thehandling of a thin film having a large area is difficult in itself, andit is very difficult to maintain alignment across the large area.

If light having a wavelength of 1.3 to 1.5 μm, like near-infrared lightfor optical communications, is controlled, the thickness of each layerin a direction of deposition may be 0.3 to 0.5 μm. Thus, the existingmethod described above can be useful. However, if the element isconsidered for use with visible light, the thickness of each layer forabout 0.4 μm equivalent to a wavelength of blue light must be 100 nm orless. Therefore, control and fabrication by the conventional method isdifficult. In 3D photonic crystals, the thickness of each layersensitively influences the optical performance even for near-infraredlight in 3D photonic crystals. Thus, it is important to reduce thethickness of layers and provide high accuracy either in terms ofimproving accuracy or purposely adjusting the thickness of each layerfinely to obtain high optical performance. In the conventionaltechnique, it is difficult to provide a thickness on the order of 1 to10 nm across a large area of 100×100 mm².

In applications in which photonic crystals are used for routing elementsfor three-dimensional optical wiring and optical communications, notonly a material should be arranged periodically, but also non-periodicstructural parts called defects are introduced at desired positions toimprove functions. In the conventional method, however, introduction ofdefects is difficult in itself, and it is difficult to perform positioncontrol of such defects across a large area.

For example, the self-cloning method has a problem in that it isdifficult to introduce defects freely. Thus, functions cannot beimproved.

As described above, the prior art described above cannot fully meet thestrict requirements for 3D photonic crystals and devices and systemsusing the 3D photonic crystals.

DISCLOSURE OF THE INVENTION

According to an aspect of the present invention, there is provided amethod for production of an optical element comprising the steps of:

(A) forming a first layer on the surface of a first substrate byepitaxial growth or micropore-making;

(B) forming a porous layer on the surface of a second substrate;

(C) forming a second layer on the surface of the porous layer byepitaxial growth or micropore-making;

(D) bonding the first layer and the second layer to each other; and

(E) separating the second substrate from the second layer at the porouslayer.

The method preferably further comprises step (F) of forming a refractionindex distribution pattern on the first layer after step (A).

The method preferably further comprises step (G) of forming a refractionindex distribution pattern on the second layer after step (C).

The steps (B) to (E) are preferably repeated with the outermost layer onthe first substrate as the first layer after step (E). The refractionindex distribution pattern formed on the second layer is preferably apattern having a periodic refraction index distribution formed in atleast one direction as a result of repeating steps (B) to (E). Thesecond substrate in step (B) in a cycle is preferably reused as thesecond substrate in step (B) in a following cycle. The method preferablyfurther comprises a step of placing a light emitting element layerbetween layers.

Step (B) is preferably a step of forming a porous layer comprised of twolayers different in porosity and the separating in step (E) is carriedout at the boundary of the two layers.

According to another aspect of the present invention, there is provideda method for production of an optical element comprising the steps of:

(a) forming alternating porous layers and microporous layers on a firstsubstrate;

(b) forming a refraction index distribution pattern on the microporouslayers collectively;

(c) bonding a second substrate to the porous layer or microporous layerbeing the outermost layer;

(d) separating a pair of microporous layers spaced by the porous layerat the porous layer;

(e) shifting the separated microporous layers from each other along anin-plane direction, and bonding the shifted microporous layers to eachother; and

(f) repeating the steps (d) and (e) for each porous layer.

According to still another aspect of the present invention, there isprovided an optical element in which layers having refraction indexdistribution patterns formed thereon are deposited to form athree-dimensional periodic distribution of refraction index,

wherein a period of the refraction index in a direction of deposition isdetermined by the thickness of the plurality of layers and a sequence ofrefraction index distribution patterns of the deposited layers. Therein,a light-emitting layer is preferably placed between the layers to form aphotonic crystal laser. A light-emitting layer having in the directionof deposition a plurality of periodic refraction index distributionsdifferent from each other and emitting a light of a wavelengthcorresponding to each period is more preferably placed two-dimensionallyalong the layer.

According to a further aspect of the present invention, there isprovided an optical device comprising:

a light source layer emitting a light of a specific wavelength; and

photonic crystals formed on the light source layer and including a lightemitting layer, which receives the light emitted from the light sourcelayer and emits a light of a wavelength different from a wavelength ofthe received light,

wherein the photonic crystals are photonic crystals in which thewavelength of the light emitted from the light source layer is out of aphotonic band gap and the wavelength of the light emitted from the lightemitting layer is within the photonic band gap. A plurality ofphotonic-crystal layers are preferably stacked, and the wavelength of alight emitted from the light emitting layer in a lower photonic crystallayer is out of a photonic band gap of an upper photonic crystal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C and 1D are schematic diagrams showing a method forfabrication of three-dimensional photonic crystals with Si of Example 1;

FIGS. 2A, 2B, 2C and 2D are schematic diagrams showing one example of aconfiguration of patterns of layers of Example 1;

FIG. 3 is a schematic diagram showing one example of a configuration oflarge area three-dimensional photonic crystals of Example 1;

FIGS. 4A, 4B, 4C and 4D are schematic diagrams showing another exampleof a configuration of patterns of layers of Example 1;

FIG. 5 is a schematic diagram showing another example of a configurationof large area three-dimensional photonic crystals of Example 1;

FIGS. 6A, 6B and 6C are schematic diagrams showing another example of aconfiguration of patterns of layers of Example 1;

FIG. 7 is a schematic diagram showing another example of a configurationof large area three-dimensional photonic crystals of Example 1;

FIGS. 8A and 8B are schematic diagrams showing an example in which oneunit of a refractive index distribution pattern of three-dimensionalphotonic crystals is formed in a multilayer structure;

FIGS. 9A, 9B, 9C and 9D are schematic diagrams showing a method forfabrication of three-dimensional photonic crystals of GaAs on Ge ofExample 3;

FIGS. 10A, 10B, 10C and 10D are schematic diagrams showing a method forfabrication of three-dimensional photonic crystals using microporous-Siof Example 3;

FIG. 11 is a schematic sectional view showing one example of ananodization method for formation of porous-Si layers of Example 3;

FIGS. 12A and 12B are graphs showing a change of an injected currentwith time in an anodization process;

FIG. 13 is a schematic sectional view showing one example of a methodfor collectively anodizing a plurality of wafers;

FIG. 14 is a schematic diagram showing one example of a configuration oflarge area three-dimensional photonic crystals of Example 3;

FIGS. 15A, 15B, 15C and 15D are schematic sectional views showing oneexample of a method for collectively fabricating multiple porous-Silayers;

FIGS. 16A, 16B, 16C and 16D are schematic sectional views showing oneexample of a method for collective alignment of multiple microporous-Silayers;

FIGS. 17A and 17B are schematic diagrams showing one example of aconfiguration of a photonic crystal laser of Example 4;

FIG. 18 is a schematic diagram showing another example of aconfiguration of the photonic crystal laser of Example 4;

FIG. 19 is a schematic plan view showing one example of a configurationof a display using photonic crystals of Example 5;

FIG. 20 is a schematic sectional view showing one example of aconfiguration of a display using photonic crystals of Example 5;

FIGS. 21A, 21B and 21C are graphs showing transmittance characteristicsof two types of photonic crystals constituting a display using photoniccrystals of Example 5;

FIG. 22 is a schematic sectional view showing another example of aconfiguration of the display using photonic crystals of Example 5;

FIGS. 23A and 23B are schematic diagrams showing an example of adefective waveguide in three-dimensional photonic crystals of Example 6;

FIGS. 24A and 24B are schematic diagrams showing one example of aconfiguration of a three-dimensional optical circuit using photoniccrystals of Example 6; and

FIGS. 25A, 25B and 25C are schematic diagrams showing one example ofμTAS using photonic crystals of Example 7.

BEST MODE FOR CARRYING OUT THE INVENTION

Terms in this specification will be defined below.

The term “microporous layer” means a porous layer formed bymicropore-making. In the present invention, a microporous layer is usedas a layer where optical devices are formed therein. That is, amicroporous layer is an alternative to an epitaxial layer. On thecontrary, the term “porous layer”, which is referred to without theprefix “micro-”, means a layer that is not used as a layer for makingdevices, but is used an a sacrificial layer for separating a microporouslayer or an epitaxial layer from a substrate.

A porous layer has any pore size. It may have a pore size larger orsmaller than, or similar to, a microporous layer. However, when a porouslayer is provided just under a microporous layer for separating it froma substrate, the pore sizes of the porous layer should be different fromthe microporous layer.

Parts used in the process of forming an optical element are defined asfollows. Names of a part comprised of a substrate and a portioncomprised in the part together with the substrate are given the ordinalnumber of the substrate. For example, layers formed on the surfaces of afirst substrate and a second substrate in step A and step C are referredto as a “first layer” and a “second layer,” respectively.

The first layer and the second layer may be comprised of at least one ofa layer obtained by epitaxial growth (hereinafter referred to simply as“epitaxial growth layer” or “epi layer”) and a microporous layer.

A part of a pre-completed optical element including a substrate and alayer formed on the surface of the substrate, obtained in Step A andStep C, is referred to as a “member.” A member comprised of the firstsubstrate and the first layer is referred to as a “first member” and amember comprised of the second substrate and the second layer isreferred to as a “second member.”

A pattern formed from a periodic difference in refractive index, whichis formed on the layer, is referred to as a “refractive indexdistribution pattern.”

A layer, before it is provided with the refractive index distributionpattern, is referred to as a “non-patterned layer.”

A member comprised of the substrate and the non-patterned layer isreferred to as a “non-patterned member.”

A layer after patterning is referred to as a “patterned layer.”

A member including the substrate and the patterned layer is referred toas a “patterned member.”

A member immediately before bonding a layer thereto in Step D isreferred to as a “pre-bonded member.” Thus, if Step D is carried outwith first and second patterned members without further processing thepatterned members, the patterned members are a “first pre-bonded member”and a “second pre-bonded member”, respectively.

The term “device layer” means a layer composed of a material allowing anelectromagnetic wave, such as visible light, near-infrared light orultraviolet light (hereinafter referred to simply as “light”), to passwithout scattering, and used for an optical device in a form of a singleor laminated multilayer as a medium propagating light. Thus, eachpatterned layer having a refraction index distribution pattern ofphotonic crystals constituting an optical device is a device layer. Anexample of the optical device will be described in detail in thisdescription. If the “first layer” and the “second layer” have refractionindex distribution patterns and constitute photonic crystalsconstituting the optical device, the layers may be referred to as a“first device layer” and a “second device layer,” respectively.

One of preferred forms of the present invention is characterized bycomprising:

non-patterned member preparing steps of preparing a first non-patternedmember having a first non-patterned layer formed on a first substrate(step A) and preparing a second non-patterned member having a porouslayer for layer separation formed on a second substrate (step B) andhaving formed thereon a second non-patterned layer composed of anepitaxial growth layer and/or a microporous layer (step C);

a pattern forming step of forming a refraction index distributionpattern on at least one of the first non-patterned layer and the secondnon-patterned layer (step F and/or step G); and

a laminated structure forming step of bonding the first layer and thesecond layer to each other (corresponding to step D and referred to as“bonding step”), then performing layer separation at the porous layerfor layer separation (corresponding to step E and referred to as“separation step”) to separate the second substrate from the secondlayer, whereby a laminated structure comprised of the first and secondlayers is formed on the first substrate (see Examples 1 to 3 describedlater). That is, the non-patterned member separating steps correspond tosteps A, B and C, the pattern forming step corresponds to step F and/orstep G, and the laminated structure forming step corresponds to steps Dand E. Incidentally, porosities of the microporous layer, as well as ofthe ordinary porous layer, are physically capable of being used for sucha separation as in the separating step. If the first or second layerformed in the member preparing steps has a refraction distributionpattern that has been intended, the member preparing step is a patternedmember preparing step. In this case, the pattern-forming step is carriedout in the member preparing step. For example, if a spatial frequencyfinally obtained when the first layer or second layer is formed bymicropore-making is equal to that of a spatial frequency associated withan intended refraction distribution pattern, it can be said that thefinally obtained layer is a patterned layer. Thus, the member preparingstep that is comprised of the micropores-making is the patterned memberpreparing step.

Other methods for preparation of the optical element of the presentinvention include a fabrication method characterized by comprising:

a patterned member preparing step of preparing a first patterned memberhaving ordinary porous layers and microporous layers formed in analternating manner on a first substrate, such as silicon, and having arefraction index distribution pattern formed collectively in themicroporous layer, and a second substrate; and

a laminated structure forming step of forming a laminated structure onthe second substrate by sequentially carrying out in each ordinaryporous layer a process of bonding together a top layer of the firstpatterned member and the second substrate, then separating the bondedmaterial by the ordinary porous layer, removing the ordinary porouslayer used for the separation, and bonding together microporous layersabove and below the ordinary porous layer with the microporous layersshifted from each other by an appropriate distance along an in-planedirection (see fabrication method in FIGS. 15A to 15D).

Other methods for fabrication of the optical element of the presentinvention include a method characterized in that different types ofmicroporous layers are formed in an alternating manner on a substrate,such as silicon, refraction index distribution patterns produced by adifference in refraction index are formed collectively on microporouslayers, and microporous layers are shifted from one another along anin-plane direction to form a laminated structure (see fabrication methodin FIGS. 16A to 16D).

The laminated structure formed by each method for fabrication of theoptical element described above has an almost periodic refraction indexdistribution in at least one direction. In this way, photonic crystalsare realized.

Further, the following aspect is possible.

The substrate and the epitaxial growth layer and/or microporous layerare made of the same material, such as silicon (see Examples 1 and 2described below).

The epitaxial growth layer and/or microporous layer and the substrate ofthese materials have a similar lattice constant and/or linear expansioncoefficient of crystals, the substrate is made of germanium or the like,and the epitaxial growth layer and/or microporous layer are made ofGaAs, GaP or the like (see Example 2 below).

The layer on the substrate is an epitaxial growth layer that has beenmade porous. Pore-making is achieved by anodization or the like. Inanodization, a voltage is applied to pass a current, and pore-makingproceeds at the deepest part of the pore. The larger the current densityat which anodization is performed, the rougher the porous structure thatis obtained. Methods for fabrication of a porous layer composed of aplurality of porous layered areas having different porosity degrees,such as a porous layer for layer separation, include a method in whichvoltages allowing large and small two current densities to be obtainedare applied for a time period corresponding to a predetermined thicknessof porosity, as shown in FIG. 12A. Particularly, if the microporouslayer is formed by anodization, it is important that a void contentgiving a required refraction index is set, and a voltage giving acurrent density allowing the void content to be obtained is applied foran appropriate time period.

FIG. 11 is a schematic diagram showing one example of an apparatusconfiguration for forming the surface of a wafer into a porous layer,i.e., an ordinary porous layer or microporous layer by anodization. InFIG. 11, a wafer 1201 is held such that the surface thereof is immersedin an HF solution 1202. The holding of the wafer 1201 is done by a lowersupport 1205 and an upper support 1206 via an O ring 1203 and a Pt planeelectrode 1204. A cistern for HF leading to the wafer 1201 is formed onthe upper support 1206 and is filled with the HF solution. A Pt meshelectrode 1208 is placed in the HF solution 1202. The Pt plane electrode1204 is connected to an anode 1207 and the Pt mesh electrode 1208 isconnected to a cathode 1209, and a predetermined voltage is applied tothe surface of the wafer 1201 via the HF solution 1202 to obtain apredetermined current density.

The method in accordance with the present invention can be applied toform various optical elements.

A laser can be fabricated by introducing a laser medium into apredetermined layer in the laminated structure (see Example 4 describedbelow). In the predetermined layer in the laminated structure, a lasermedium can be introduced at and around the position of a non-periodicpattern with the non-periodic pattern included at a predeterminedin-plane position.

At least one of a red light source, a green light source and a bluelight source can be constituted by such a laser system to fabricate adisplay apparatus (see Example 5 described below).

A three-dimensional optical wiring, optical circuit or the like can beprovided by using a three-dimensional waveguide formed by mutuallyconnecting a deposition direction defective waveguide passing through aplurality of layers and formed in the direction of deposition and anin-plane defective waveguide formed on the layers in an in-planedirection (see Example 6 described below).

A laser sensor system can be fabricated by providing at least one lasersensor formed using the laser system described above and characterizedby detecting an oscillation state of a laser, forming a very smallchannel system near the laser sensor, and detecting information about afluid flowing through the very small channel (see Example 7 describedbelow).

In view of the problems described above, the optical element of thepresent invention is characterized in that a plurality of layersincluding layers having at least one of an epitaxial growth andmicropore-making type are deposited to form the optical element as athree-dimensional structure, and a refraction index distribution patternproduced by a difference in refraction index is formed in the plane ofat least one layer. In this case, in the refraction index distributionpattern, the three-dimensional structure can have almost a periodicrefraction index in at least one direction. One unit of the refractionindex distribution pattern in the direction of deposition may be formedby linkage of refraction index distributions formed in planes of aplurality of layers deposited in the direction of deposition (see FIGS.8A and 8B).

The optical element for display of the present invention ischaracterized by comprising:

a light source portion capable of emitting independently at least twobeams including a beam for display and a beam for excitation in acontrollable manner; and

a photonic crystal portion including a light emitting element portionformed on the light source portion,

wherein the light emitting portion is arranged such that it can receivethe beam for excitation and oscillate, and the photonic crystal portionis formed such that it allows the beam for display to pass but theoscillation wavelength of the light emitting element portion is within aphotonic band gap (see Example 5 described below).

The member preparing step and the laminated structure forming step canbe repeated two or more times to form a laminated structure. Usually,about 8 layers are required for light blocking control using thephotonic band gap. Thus, if a defect is formed at the center to performlight containment control, for example, a laminated structure having 16layers in total, i.e., 8 layers on each of both sides of the defect, isrequired.

As described above, the present invention realizes an optical elementhaving a three-dimensional laminated structure having a refraction indexdistribution pattern using layer separation between different porouslayers. In this way, by using a method for fabrication of photoniccrystals, nanophotonic elements and the like, using a device layerrelocation technique, the optical element can be realized with ahigh-functional three-dimensional optical element or system having arelatively large area and having defects and the like introduced withoutrestraint, and fabricated at a lower cost than was previously possible.The optical element can be realized with a high-functional device in avisible light range using, for example, a nanophotonic element made tohave a micropourus structure by anodizing the epitaxial growth layer orthe like of the present invention.

Specific embodiments will be described below with reference to theaccompanying drawings to specify the embodiments of the presentinvention.

EXAMPLE 1

In this Example, a nanophotonic element is produced by using silicon asa material for three-dimensional photonic crystal.

The first step of this Example is comprised of a step of preparing thefirst member and a step of forming a pattern.

Of the first steps shown in FIG. 1A, a silicon layer 102 having a porousstructure of two layers having different porous densities is formed onan Si substrate 101 by anodization in the step of preparing the firstmember. By changing the conditions of anodization, two layers havingdifferent porous densities can be formed. A monocrystal silicon layer103 (which does not have to be a monocrystal in a strict sense; the sameapplies to monocrystals used elsewhere) is grown thereon as the firstlayer of the present invention by epitaxial growth to a pre-determinedthickness. Since the silicon layer 103 formed by epitaxial growth isused as a device layer after the step for forming a pattern, thethickness is determined according to the optical element to be formed.

Then, as the step for forming a pattern, patterning is conducted byphotolithography on the epitaxially grown silicon layer 103 to form apre-determined spatial pattern. Since this pattern functions as a layerthat creates a periodic refractive index distribution of the 3D photoniccrystal, a semi-periodic structure is formed in at least one direction.In this Example, the first patterned layer 103 forming a refractiveindex distribution pattern is a layer used in the form of a multi-layerlaminate for a nanophotonic element, which is an optical element, thatis, a device layer, and therefore, the layer is referred to as the“first device layer.” A non-periodic structure, which is a defect forthe periodic structure, can be introduced at a pre-determined position.For patterning, various techniques other than using light, such as EB(electron beam) lithography, evanescent near-field lithography, x-raylithography, ion beam lithography, nanoimprint and patterning combiningnanoimprint and anodization can be used.

In this Example, examples of the pattern of the epitaxially grownmonocrystal silicon layer, which is the device layer, are shown in FIGS.2A to 2D. Here, the pattern of FIG. 2A was selected as the pattern ofthe monocrystal silicon layer 103. As for the size of the first devicelayer 103, the thickness of the layer was about 0.25 μm, assuming thatthe wavelength of the light to be used was 1.5 μm. The period of thepattern was about 0.7 μm. The first member was formed in this manner.These procedures so far are in the step for preparing the first member.In the case of using visible light, the sizes are about ½ to ¼ of thevalues, but the above-mentioned structure of the first member can beused as it is.

Next, as the step for preparing the second member, which constitutes thefirst half of the second step of this Example, the same procedures as inthe step for preparing the first member are carried out for the secondsubstrate 104 comprising Si. The obtained second member is shown in FIG.1B. In the figure, the silicon layers constituting the formed porouslayer 111 and having a porous structure of two layers having a differentporous density are represented by reference numerals 105 and 106. Themonocrystal silicon layer epitaxially grown on the porous silicon layeris represented by reference numeral 107. Next, as the second half of thesecond step, patterning different from the pattern formed on the firstdevice layer 103 in the first step is conducted on the monocrystalsilicon layer 107 by photolithography to form a pre-determined spatialpattern. In this Example, the pattern shown in FIG. 2B was formed on thelayer 107. The patterned monocrystal silicon layer 107 is also a devicelayer and is referred to as the “second device layer” in this Example.

The first part of the third step includes a bonding step in the step forforming a laminate structure. As shown in FIG. 1B, the device layers onwhich different patterns are formed are properly aligned, placedopposite each other and jointed by melt adhesion or lamination. Forbonding, direct bonding can be employed. As an example of the directbonding, a natural oxide film is removed by washing the silicon surfaceby a mixture of H₂O₂ and H₂SO₄; the smooth surfaces having hydroxylgroups are laminated at room temperature to create a hydrogen bond;dehydration condensation is carried out on the interface by heating to ahigh temperature of about 500° C.; and the remaining oxygen is diffusedby increasing the temperature to about 1,000° C. to achieve a strongbonding of silicon atoms.

The second part of the third step is a separating step in the step forforming a laminate structure. The porous layer 111 is separated in adirection parallel to the layer so that the jointed device layer is lefton one of the substrates. The separation is conducted by water jet(hereinafter referred to as WJ) toward the boundary between the twoporous layers 105 and 106 having different porous densities, as shown byan arrow in FIG. 1B. In such a boundary, mechanical stress remains dueto the mismatch of the lattice constant caused by the difference in theporous density. Accordingly, upon the application of the WJ, release ofthe remaining stress and separation of lattices having different latticeconstants proceed half spontaneously.

The fourth step includes a removing step and a smoothing step asdescribed in the following. A porous layer remains on the second devicelayer after the separating step, as shown in FIG. 1C. Thus, the porouslayer is removed by selective etching. As shown in FIG. 1D, theremaining porous layer 106 on the surface is removed while leaving thesecond device layer 107. This step is referred to as the “removingstep.”

Then, the surface of the second device layer 107 from which the porouslayer is removed is smoothed. The step is referred to as the “smoothingstep.” The surface of the second device layer 107 from which the porouslayer is removed is subjected to an annealing treatment in 100% H₂ at1,050° C. and smoothed to the atomic level.

According to these four steps, a structural body in which twodifferently patterned epitaxially grown silicon layers on wafer arejointed and laminated can be obtained. The substrate 104 separated byseparation of the porous layers can form a porous layer and anepitaxially grown layer again, which means that it is reusable. Thesubstrate, which has been separated and is intended for reuse, thesecond substrate 104 in this Example, is referred to as the “seedsubstrate”.

From the fifth step onward, by using the structural body obtained by thefirst to the fourth steps as the new first member and repeating thesecond step through the fourth step, the number of laminated devicelayers can be increased. Layers are laminated by this method to apre-determined number optically necessary for a photonic crystal.Specifically, the second member is prepared using the seed substrate 104and subjected to patterning. The resulting structure is jointed to thefirst member, which is a wafer having a plurality of device layerslaminated so far, as shown in FIG. 1B, and then separation, removal ofthe porous layer and smoothing are carried out as in FIGS. 1C and 1D.For the pattern in the case of repeating, each device layer islaminated, for example, in the order of FIGS. 2A, 2B, 2C, 2D, 2A, 2B, .. . , and several embodiments are available for repeating.

It is possible that in the first step, the porous layer 102 of the firstmember is formed to have two layers with different porous densities asin the second member, several layers are laminated by repeating thesecond step to the fourth step, the second member is finally jointed andthen the porous layer 102 is separated to complete the separation. Inthis case, the porous layer 105 for the separating step may not beformed on the second member.

After going through all the steps for obtaining a required number oflayers, a three-dimensional photonic crystal as schematically shown inFIG. 3 is obtained. Since epitaxial growth is employed as a means forforming a thin layer for fabricating a three-dimensional photoniccrystal in this Example, the thickness of each layer can be reduced tobe on the order of 10 nm, providing a large area of a photonic crystalfor short-wave light, such as green and blue visible light andultraviolet light. In addition, each layer has a smooth surface at theatomic level, i.e., on the order of 1 nm. According to the achievementof such smoothness, a high-quality, large area photonic crystal that issubstantially free of the scattering loss of light and the deteriorationin quality due to an unexpected defect can be achieved.

The pattern of each device layer in this Example, as shown in FIGS. 2Ato 2D, can be changed accordingly in consideration of the object, thematerial and the apparatus for patterning. For example, various laminatestructures can be prepared, such as a diamond structure (FIG. 5) inwhich each layer constituted by rectangular parallelepipeds arranged ina checkered pattern shown in FIGS. 4A to 4D is laminated and a periodicstructure (FIG. 7) in which each layer having cylindrical pores isarranged in a triangular lattice shown in FIGS. 6A to 6D. In addition, arefractive index distribution pattern can be easily created by shiftingthe pore of the pattern from the periodic position, changing the size,eliminating pores locally or by specifying the position of a defectthree-dimensionally. As described above, according to the productionmethod of this embodiment, the pattern of each layer can be specifiedand formed independently. Accordingly, a highly flexible laminatestructure in which photonic crystals having two periodic refractiveindexes are mixed is easily accomplished, and a high performance opticalelement and optical system using silicon can be achieved.

It is described in this Example that the pattern formed on thenon-patterned layer 107 in the step for forming a pattern of the secondstep is different from the pattern of the first device layer 103. Inpractice, however, the difference in the patterns between the adjacentlayers in the laminate structure may be created by displacing the samepattern in the in-plane direction and laminating as illustrated in FIGS.4A to 4D and 6A to 6D. Alternatively, the difference may be created byincluding a layer without a refractive index distribution pattern.

The technique of relocating a high-quality thin film crystal layer,which is a microporous crystal layer prepared from an epitaxial crystallayer or a monocrystal, from the first member to the second member canbe applied to a thin film having a thickness of 1 nm to 10 nm. Utilizingthis technique, one periodic unit of the periodic refractive indexdistribution of the photonic crystal in the lamination direction may beconstituted by a laminate structure of a plurality of epitaxially grownlayers. This structure enables designing of the spatial frequency in thelamination direction by changing only the detail of the periodic unitwhile maintaining the designed pattern. FIG. 8A shows a periodic unitpattern in the lamination direction. FIG. 8B shows an approximatedpattern of FIG. 8A created by laminating a plurality of layers. Toapproximate a ball having a diameter of 200 nm by a multi-layerstructure, an approximate pattern can be formed with a high precision bylaminating 20 layers of thin films having a thickness of 10 nm.

EXAMPLE 2

The second Example of the present invention is described below withreference to FIGS. 9A to 9D. In this Example, a nanophotonic element isproduced by using GaAs, which is a compound semiconductor, as a materialof a three-dimensional photonic crystal and using Ge as a seedsubstrate.

The first step is comprised of a step for preparing the first member anda step for forming a pattern. In the step for preparing the firstmember, Ge layer 802 having a porous structure is formed on the firstsubstrate 801 comprising Ge by anodization, as shown in FIG. 9A. Thefirst member is prepared by forming the first layer 803 comprisingmonocrystal GaAs thereon by epitaxial growth to a pre-determinedthickness.

Next, the first patterned layer is obtained by conducting patterning fora pre-determined periodic refractive index distribution on the firstlayer 803 by photolithography to form a layer, which constitutes the 3Dphotonic crystal, as in the step for forming a pattern of the firststep. The obtained layer is the first device layer. A non-periodicstructure, which is a defect for the periodic structure, can beintroduced at a pre-determined position. Various patterning methodsdescribed in Example 1 can be accordingly used.

The second step is comprised of a step for preparing the second memberand a step for forming a pattern. As the step for preparing the secondmember, a two-layer structure porous layer 811 comprising porous layers805 and 806 having different porosities is formed on the secondsubstrate 804 comprising Ge by anodization, and the second member isprepared by forming the second layer 807 comprising monocrystal GaAsthereon by epitaxial growth to a pre-determined thickness. As the stepfor forming a pattern in the second half of the second step, patterningfor a pre-determined periodic refractive index distribution is conductedon the second layer 807 to form a layer, which constitutes the 3Dphotonic crystal. This pattern may be the same as the pattern formed onthe first layer in the first step provided that the period is completelythe same, whereas a pattern slightly different from that of the firstlayer is to be formed on the second layer if each pattern contains quitea few defects. The second layer on which a pattern is formed is referredto as the “second device layer.”

The first half of the third step includes a bonding step. As shown inFIG. 9B, the device layers are properly aligned, placed opposite eachother and jointed by melt adhesion or lamination. For bonding, directbonding described in Example 1 may be used.

The second half of the third step is a separating step. As shown in FIG.9C, the porous layer 811 is separated so that the jointed device layeris left on one of the substrates. The separation is conducted by waterjet as described in Example 1 toward the boundary between the two porouslayers 805 and 806 having a different porous density (shown by an arrowin FIG. 9B).

The fourth step includes a removing step and a smoothing step. Here, aporous layer remaining on the second device layer after the separatingstep is removed by etching, as shown in FIG. 9C. The surface of thesecond device layer after removing the porous layer is smoothed to theatomic level by an annealing treatment.

According to these four steps, a structural body in which two patternedepitaxially grown GaAs layers are jointed and laminated on wafer via aporous layer can be obtained. The separated GaAs layer is reusable as inExample 1.

From the fifth step onward, it is possible to increase the number of thedevice layers to be laminated by repeating the second to the fourthsteps in the same manner. Layers are laminated to a pre-determinednumber optically necessary for a photonic crystal.

After going through all the steps for obtaining a required number oflayers, a three-dimensional photonic crystal as schematically shown inFIG. 3 is obtained. GaAs of each layer also has a smooth surface due tothe precision of epitaxial growth in the three-dimensional photoniccrystal of this Example. Namely, smoothness at the level of 1 nm ismaintained. Therefore, a high-quality product that is substantially freeof scattering loss of light in the photonic crystal and deterioration inquality due to an unexpected defect can be achieved.

The in-plane pattern of each layer can be specified independently inthis Example as well. Accordingly, a highly flexible laminate structurein which photonic crystals having two kinds of periods are mixed iseasily accomplished and excellent performance of an optical element andan optical system using GaAs, which is a direct transition opticalsemiconductor, can be achieved.

In this Example, GaAs was used as the constituent material of thephotonic crystal, but materials suitable for a Ge substrate, such asGaP, AlAs and AlP, which have a similar crystal lattice constant and/orlinear expansion coefficient, may be used depending on conditions, suchas the film thickness.

A structure shown in FIG. 7 can be fabricated using the pattern shown inFIGS. 4A to 4D and FIGS. 6A to 6D in this Example as well. In addition,a structure shown in FIGS. 8A and 8B can also be achieved.

EXAMPLE 3

In Example 3, both the porous layer and the device layer thereon areformed by anodization without the step of epitaxial growth. Athree-dimensional photonic crystal having small light absorption in ashort-wave range, such as short-wave near-infrared light, visible lightand ultraviolet light is provided.

The first step in this Example is comprised of a step for preparing thefirst member and a step for forming a pattern. In the step for preparingthe first member, silicon layer 1103 comprising two porous layers 1102and 1104 having different porosities is formed on an Si board 1101 byanodization using an apparatus shown in FIG. 11, as shown in FIG. 10A.Of the two layers, the upper layer, i.e., the layer 1104 on the surface,is a microporous layer that transmits light. The microporous layer 1104has, for example, a typical construction of a pore diameter of about 1nm to 10 nm and a porosity of about 20% to 80%. This construction makesthe refractive index of the layer similar to that of air and improvesthe transmittance of visible light.

Of the two layers, the lower porous layer 1102 is supposed to have aporous structure different from that of the microporous layer so thatthe WJ can separate the microporous layer 1104. The types of porousstructure may be, for example, macro, meso or micro, and are notlimited.

With respect to the relationship between time and the amount of theapplied voltage in this Example, voltages allowing large and small twocurrent densities to be obtained are applied for a time periodcorresponding to a predetermined thickness of porosity, as shown in FIG.12A. Thereby, a plurality of porous layered areas having differentporosity degrees are formed at the surface. As each of the voltages isapplied to pass a current, pore-making proceeds at the deepest part ofthe pore. The larger the current density at which anodization isperformed, the rougher is the obtained porous structure. Accordingly, asshown in FIG. 12A, the lower layer has a rougher porous structure formedby the larger current density and the uppermost layer consists of amicroporous layer formed by the lesser current density. It is importantfor forming the microporous layer as the uppermost surface layer to seta void content yielding a required refraction index and to apply avoltage resulting in a current density allowing the void content to beobtained for an appropriate time period. This is because the uppermostsurface layer is used as a main portion of an optical device.

For creating pores in this Example, a plurality of wafers 201 can besubjected to anodization all together by using an apparatus shown inFIG. 13. In FIG. 13, reference numeral 202 denotes a wafer holder,reference numeral 203 denotes an O-ring, reference numeral 204 denotes asuction part, reference numeral 205 denotes an HF solution, referencenumerals 206 a and 206 b denote a platinum electrode, reference numeral208 denotes an anodization bath and reference numeral 209 denotes aholder groove.

In this Example, the constitution for conducting the anodization is notlimited to the formation described above, but various general methodscan be accordingly used.

Next, the first patterned layer is obtained by conducting patterning fora pre-determined periodic refractive index distribution on themicroporous layer 1104 by photolithography to form a layer, whichconstitutes the 3D photonic crystal, as shown in FIG. 10B, as the stepfor forming a pattern. The obtained first patterned layer is the firstdevice layer. A non-periodic structure, which is a defect for theperiodic structure, can be introduced at a pre-determined position.Various patterning methods described in Example 1 can be accordinglyused. The pattern shown in FIG. 2A was employed as the pattern of thefirst device layer of this Example.

As the step for preparing the second member, which constitutes the firstpart of the second step of this Example, the same procedures as in thestep for preparing the first member are carried out for the secondsubstrate 1105 comprising Si to form a silicon layer 1111 comprising aporous layer 1106 and a microporous layer 1107. Next, as the second halfof the second step, the microporous layer 1107 is subjected topatterning and the second patterned layer having the pattern of FIG. 2Bis obtained. The obtained second patterned layer is the second devicelayer.

Next, as the first sub-step of the third step, the first device layer1104, which is a microporous layer forming a pattern, and the seconddevice layer 1107 are arranged facing each other and being jointed, asshown in FIG. 10C. For bonding, direct bonding described in Example 1may be used.

The second sub-step of the third step is a separating step. As shown inFIG. 10D, one of the porous layers and the device layer, which are theporous layer 1106 and the microporous layer 1107 in this case, areseparated so that the first and second device layers comprising themicroporous layer are left on one of the substrates. The separation isconducted by the WJ as described in Example 1 toward the boundarybetween the two layers having different porous densities (shown by anarrow in FIG. 10C).

It is preferable to conduct the anodization under conditions that canmake the separated surface as smooth as possible, for example, at a lowtemperature.

After the separation, the surface of the microporous layer 1107 can besmoothed by an annealing treatment.

According to these three steps, a structural body in which two patternedmicroporous layers 1104 and 1107 are jointed and laminated on the wafervia the porous layer 1102 can be obtained. From the fourth step onward,it is possible to increase the number of the microporous layers, i.e.,the device layers to be laminated by repeating the second step and thethird step, to obtain a number of layers optically necessary for aphotonic crystal.

Through the above processes, a three-dimensional photonic crystalconstituted by microporous-Si, as shown in FIG. 14, can be obtained. Thethickness of each layer can be precisely adjusted to not more than 100nm by controlling the voltage in the anodization. FIG. 14 illustrates alayer having a thickness of 70 nm and a period of the periodicrefractive index distribution of 200 nm.

Since the photonic crystal of this Example uses microporous-Si as amaterial, it has a low absorption even in a visible light region. Forthe attenuation coefficient, absorption is lower than usual Si, on theorder of about 104. Thus, according to this Example, a high performancethree-dimensional photonic crystal that has a small absorption,particularly in a visible light region, can be obtained.

In this Example, the step for making the upper surface of the Sisubstrate porous was set prior to the step for forming a pattern, butthis can be done after the step for forming a pattern or after thebonding step, and the order can be selected depending on the conditions.It is also possible to change the order depending on the laminatedportion.

It is also possible to fabricate a three-dimensional photonic crystal byforming a microporous layer 1104, which is the device layer, and aordinary porous layer 1102 alternately on the first substrate and byrepeating the bonding step and the separating step using the secondsubstrate as a seed substrate. This is achieved, for example, by thesteps schematically described in FIGS. 15A to 15D. As shown in FIG. 15A,after forming a structure in which the microporous layer 1104 and theordinary porous layer 1102 are alternately laminated by anodization byapplying voltage periodically, as shown in FIG. 12B, patterning isconducted so as to perforate through the laminate structure, as shown inFIG. 15B. This structure is jointed on a wafer separately prepared, asshown in FIG. 15C, on which a porous layer may be formed as shown in thefigure or which may be a simple substrate on which nothing is formed.Then, as shown in FIG. 15D, a structure is formed by repeating the stepsof removing the ordinary porous layer by separating at the ordinaryporous layer 1102 and jointing while shifting the microporous layers1104 half a period. In this method, when the microporous layer 1104 issufficiently thin and the position is properly adjusted upon shifting,the structure of FIGS. 8A and 8B can also be prepared.

When fabricating the multilayer as mentioned above and the two porouslayers are both microporous layers, the following is possible. That is,in the separation step of FIG. 16C after the perforation patterning ofFIG. 16B, across-the-board, half-period shifting can be conductedbetween the two kinds of porous layers 1102 and 1104 by using a processfor dragging the multi-layered porous layers 1102 and 1104 alternatelyin opposite directions from the both sides. Then, by jointing as it isas shown in FIG. 16D, a microporous-Si three-dimensional photoniccrystal can be prepared.

In this Example, a material other than Si, such as GaP, AlP and AlAs,may be accordingly used in a combination with a seed substrate made ofSi or Ge. In such case, after forming a porous layer on the uppersurface of the seed substrate made of Si or Ge, an epitaxially grownlayer of GaP, AlP or AlAs is formed and the epi-layer is mademicroporous to conduct patterning thereon. The subsequent steps can becarried out as described above.

EXAMPLE 4

The fourth Example of the present invention (PC laser) is as follows.This Example illustrates a case of fabricating a laser device using thethree-dimensional photonic crystal prepared according to the method ofthe present invention.

FIG. 17A is a schematic view showing a photonic crystal laser in which alaser medium layer 1403, which is to be a sheet-type emitting layer, isprovided. In FIG. 17A, the laser medium layer 1403 is interposed betweenthe lower three-dimensional photonic crystal 1401 and the upperthree-dimensional photonic crystal 1402.

The method of making such arrangement involves two alternatives. Thatis, 1) a method of lamination from the lower photonic crystal accordingto the method described in the above-mentioned Example 1 and thenlaminating the laser medium layer 1403 as well; or 2) a method in whichthe upper and lower photonic crystals are prepared in advance accordingto the method of Example 1 and then fused with the laser medium layer1403. In this case, patterning can be conducted on the laser mediumlayer 1403 itself to incorporate a periodic structure or a periodicstructure with a defect structure easily.

A wire for current injection or a system for optical excitation, whichis not shown in the figure, is provided on the laser medium layer 1403.Based on the laser principle, laser oscillation occurs with the upperand lower photonic crystals 1401 and 1402 as a resonator, i.e., anarrowband element. For the mode of the laser oscillation, when theupper and lower photonic crystals 1401 and 1402 have a periodicstructure in which no defect is incorporated, a DFB oscillation, i.e., aphotonic band edge oscillation, is caused, where the mode has arelatively broad area, which covers a plurality of periods of thephotonic crystal. In addition, defect 1404 may be introduced on theadjacent layer contacting the laser medium layer 1403 of the upper andlower photonic crystals 1401 and 1402, as shown in FIG. 17B. In thiscase, the laser resonator functions in a local mode, i.e., anoscillation mode corresponding to one period of the photonic crystal asa whole to emit light.

The laser medium layer 1403 may be comprised of an organic dye, such ascoumarin, rhodamine, DCM and Alq3, or a host material containing a dye.In addition, a compound semiconductor containing a ternary or quaternarymixed crystal such as GaAs, InP, InGaN, InGaAs and InGaAlP, may be useddepending on the purpose. The laser medium layer may have , for example,a multiple quantum well structure or a quantum dot structure.

On the other hand, as an excitation source, electron or holetransporting materials, such as Alq3 and TPD, current injection via anelectrode, such as ITO and MgAg, and optical excitation using N₂ gaslaser, Nd:YAG higher harmonic waves and blue/ultraviolet semiconductorlaser, which are widely used in organo-electroluminescence devices, canbe accordingly used. In addition, in this Example, laser medium 1502 maybe provided in the form of a dot within the three-dimensional photoniccrystal 1501, as shown in FIG. 18, or may form a point defect resonator.FIG. 18 is a schematic view illustrating a cross-section in an in-planedirection at the position where the laser medium 1502 is located.

As described above, a three-dimensional photonic crystal laser using thedevice layer relocation technique of the present invention has beenachieved. By using a reduced loss high performance three-dimensionalphotonic crystal resonator of the present invention, a light-emittingelement capable of oscillating at a wavelength that has previouslyresulted in a large loss (e.g., green light which has been difficult ina dye laser) or in a micro mode can be achieved.

EXAMPLE 5

The fifth Example of the present invention is as follows. This Exampleillustrates a case of fabricating a color display using a photoniccrystal light-emitting element prepared according to the method of theabove-mentioned Example 4.

FIG. 19 is a schematic view of a part of the display of this Exampletaken from the direction vertical to the display surface. The displaycomprises light-emitting pixels of three primary colors of Red (R) 1601,Green (G) 1602 and blue (B) 1603, which are integrated in an array.

FIG. 20 is a schematic view illustrating a cross-sectional structure ofthe part corresponding to a pair of the pixels of the three primarycolors. In the structure of FIG. 20, a blue photonic crystal layer 1701,a green photonic crystal layer 1702 and a red photonic crystal layer1703 are formed from the bottom in the lamination direction.

As described in Examples 1 to 3, since any pattern can be formed in thelayer and the thickness of the layer is variable in the presentinvention, the structure of FIG. 20 can be fabricated by setting thepattern and the thickness in three ways and forming the layerssequentially.

Each photonic crystal layer has a photonic band gap for the wavelengthof the color, and therefore, serves as a photonic crystal laser of thatcolor. In the blue photonic crystal layer 1701, blue light is in thephotonic band gap.

FIGS. 21A to 21C show the transmittance of the green photonic crystallayer 1702 and the red photonic crystal layer 1703 at (a) R pixelposition, (b) G pixel position and (c) B pixel position. In each graph,the solid line plot 1801 indicates the wavelength-based characteristicof the transmittance of the green photonic crystal layer 1702 and thebroken line plot 1802 indicates the wavelength-based characteristic ofthe transmittance of the red photonic crystal layer 1703.

In the green photonic crystal layer 1702, green light is within thephotonic band gap, and an arrangement is made so that the blue light isoutside the band gap. Therefore, the blue light is transmitted. In thered photonic crystal layer 1703, red light is within the photonic bandgap, and an arrangement is made so that blue and green lights areoutside the band gap. Therefore, the blue and green lights aretransmitted.

In the structure of FIG. 20, the blue photonic crystal layer 1701, whichis the lowest layer, is prepared according to the method described inExample 1. The layer may be prepared by the method described in Example2 or 3. Since the blue laser is used as excitation light for red andgreen lasers in this Example, the blue photonic crystal layer 1701 hasthree blue lasers 1704 in the in-plane direction so as to make ON/OFFswitching independently by wires, which are not shown in the figure.

The green photonic crystal layer 1702 is formed on the blue photoniclayer 1701 as a three-dimensional photonic crystal in which the pitch ofthe layer pattern and the layer thickness are different from those ofthe blue photonic crystal layer and arranged to have a photonic band gapin the wavelength range of green.

The output light from the blue laser 1704 is introduced to the positions1705, 1706 and 1707 in the in-plane direction of the green photoniccrystal layer 1702. Of these, the position 1706 constitutes a greenphotonic crystal laser part. That is, the three-dimensional photoniccrystal laser fabricated according to the method of Example 4 isprovided with the oscillation wavelength being set to the wavelength ofgreen light. Excitation means in this Example is light irradiated fromthe blue laser 1704. Therefore, by the ON/OFF switching of the bluelaser, the output of the green laser can be switched ON and OFF.

Since the green photonic crystal 1702 is designed to have a band gaponly in the wavelength range of green light, the output from the bluelaser 1704 is transmitted at a high transmittance at the part 1705corresponding to the blue (B) pixel and the part 1707 corresponding tothe red (R) pixel.

Next, as in the case of the green photonic crystal layer 1702, the redphotonic crystal layer 1703 is made of the three-dimensional photoniccrystal of the present invention, and a red photonic crystal laser 1710is formed only on the red pixel portion in the in-plain direction. Thered photonic crystal laser 1710 is excited by the blue laser beamtransmitted through the green photonic crystal 1707 to output red laser.The other parts, i.e., the red photonic crystal 1709 on the green pixelpart and the red photonic crystal 1708 on the blue pixel part, transmitgreen light and blue light, respectively, at a high transmittance, asshown in the broken line plots 1802 in FIGS. 21A to 21C. At this stage,the output lights from the two blue lasers 1704 used as the excitationlight for the green photonic crystal laser part 1706 and the redphotonic crystal laser part 1710 are set to be higher in the intensitythan that of the output light from the blue laser 1704, which istransmitted and utilized as it is, as represented by the width of thearrows in FIG. 20.

In this way, R (red light emission) 1713, G (green light emission) 1712and output light B (blue light emission) 1711 corresponding to eachpixel of R, G and B are obtained. These are mixed to form the pixel of acolor display and by integrating these in the in-plane direction, adisplay having a large number of pixels, as shown in FIG. 19, can befabricated.

The output mode of the laser, i.e., light distribution(direction-dependent light intensity distribution) in each of thephotonic crystal laser parts 1704, 1706 and 1710, can be adjusted bysetting the photonic crystal as a resonator. In addition, by setting arefractive index distribution of the photonic crystal on the parts 1705,1707, 1708 and 1709, which are used as transmission parts in thephotonic crystal, some factors of an optical element, such as focusingof light and collimation, can be imparted as a passive transmissionelement.

As the excitation means for R, G and B, the structure as shown in FIG.22 may be employed. That is, in FIG. 22, a blue light source 1901 is alight source, which illuminates the area covering a plurality of pixelson which an optical shutter layer 1902 is formed. The optical shutterlayer 1902 has a shutter 1903, which blocks the blue light from the bluelight source 1901, and a variable shutter 1904. The variable shutter1904 is designed to switch the transmittance of the blue light per pixelR, G and B independently. A device similar to that of FIG. 20 is formedon the upper layer of the optical shutter layer 1902. In thisconstruction, it is essential that the B pixel parts 1905 and 1906 ofthe green photonic crystal layer and the red photonic crystal layer aredesigned so as to make the light distribution of the blue lightcomparable to the display output. The same applies to the G pixel partof the red photonic crystal layer. As the variable shutter 1904, variousshutters including a movable shutter using microelectromechanics (MEMS)and a variable optical shutter, such as a liquid crystal, can be used.

Additionally, in this Example, the outputs from the light source at thelowest layer and the excitation source are varied for RGB (representedby the width of the arrows in the figure as mentioned above), or thetransmittance can be adjusted at the transmitted photonic crystal partin consideration of the balance of the amount of RGB light and the laserefficiency. The alignment of pixels 1601, 1602 and 1603 shown in FIG. 19can be optionally changed.

EXAMPLE 6

The sixth Example of the present invention is as follows. This Exampleillustrates an application of the three-dimensional photonic crystal ofthe present invention to an optical wire or an optical circuit byintroducing thereto a three-dimensional waveguide defect. Here, a defectwaveguide in the lamination direction is also used in a combination, inaddition to the widely known in-plane defect waveguides.

FIGS. 23A and 23B are schematic views illustrating a case of creating athree-dimensional defect waveguide of this Example. FIG. 23A shows acase of a defect waveguide, which is a component thereof in thelamination direction, and FIG. 23B shows a schematic view of an in-planedefect waveguide, which is another component.

In FIG. 23A, reference numeral 2001 is the three-dimensional photoniccrystal fabricated according to the method of the present invention, anda schematic cross-section thereof in the lamination direction is shown.Two defect waveguides in the lamination direction are formed at thecenter of the cross-section, and the defect waveguide 2003 in thelamination direction is formed by widening the pore size of the periodicpore at a specified portion in each layer, while the defect waveguide2002 in the lamination direction is formed by closing the periodic poreat a specified portion in each layer.

In the waveguide 2003 of this Example, since the same periodic patternis formed on each layer and the widening of the pore size is alsoconducted on the same portion, as shown in FIG. 23A, and the photoniccrystal is formed according to the method of shifting each layer havingthe same pattern half a period, the cost for forming the waveguidepattern in accordance with this Example is relatively low.

In this way, as in the case of FIG. 23A, introduction of the defectwaveguide in the lamination direction is easy in the three-dimensionalphotonic crystal of this Example. By a suitable combination of, forexample, coupling with a defect waveguide 2005 in the plane 2004direction of a general photonic crystal, as shown in FIG. 23B, athree-dimensional waveguide network can be created in thethree-dimensional photonic crystal with a high degree of flexibility.

While FIG. 23A illustrates the case in which the defect waveguide in thelamination direction is formed in parallel with the laminationdirection, a three-dimensionally oblique waveguide, i.e., a waveguideextending to both the lamination direction and the in-plane direction,can also be formed by shifting the in-plane position little by littlebetween the layers.

In addition, according to the method of the present invention, since anepitaxially grown layer can be used for each layer of the photoniccrystal, the thickness can be set on the order of 10 nm, which isextremely thin, as described above. In that case, the layer thicknesscan be smaller than the periodic length required for a photonic crystal.Therefore, one period may be comprised of a plurality of layerthicknesses (see explanation of FIGS. 8A and 8B above). Accordingly, thedefect waveguide in the lamination direction that is as fine as not morethan one period can also be arranged, achieving most appropriate defectwaveguide structure.

Next, in FIGS. 24A and 24B, a case of an optical circuit using theabove-mentioned three-dimensional waveguide is shown. FIG. 24A is aschematic view showing a case of fabricating an optical-optical switchcircuit comprising two three-dimensional waveguides and athree-dimensional optical non-linear material introduced at theintersection thereof. In FIG. 24A, two three-dimensional waveguides 2102and 2103 are formed inside the three-dimensional photonic crystal 2101.The layer 2104 at the intersection thereof is different from otherlayers, and a three-dimensional optical non-linear material 2105 isintroduced at the intersection. Since the three-dimensional opticalnon-linear material 2105 has a so-called cross-phase modulation effect,the phase of the light 2107 passing through the other light waveguide2103 is modulated in proportion to the intensity of light 2106 incidentupon the non-linear material through the light waveguide 2102.

Accordingly, by converting the phase of light passing through the lightwaveguide 2103 to an intensity signal by using a phase detector, such asa Mach-Zehnder interferometer, which is not shown in the FIG., switchingof the intensity occurs dependent on the light waveguide 2102.

FIG. 24B is a schematic view illustrating a three-dimensionalcombination of a plurality of such waveguides, describing only theposition of the waveguides. Three waveguides 2109 intersect with twowaveguides 2108 via the optical non-linear material 2110 at theintersection, and by the intensity of light passing through thesewaveguides switching occurs mutually. A more complicated light circuitnetwork is formed by combining such circuit.

As the three-dimensional optical non-linear material, various materialsmay be used, including a semiconductor exciton or a material using a lowdimensional effect thereof, such as a quantum dot and a quantum well, amaterial using fine metal particles and oxide crystals, such as LiNbO₃and TiBaO₃.

In addition, since the element comprises a photonic crystal in thisExample, the effects (low group velocity light, resonator QED etc.) canbe utilized by forming a photonic crystal structure, which reduces thegroup velocity of the guided light or a small photonic crystal resonatornear the position where a non-linear material is incorporated. In thisway, various characteristics of the photonic crystal, such as enhancedswitching effect due to the increased non-linear mutual operation time,can be easily utilized.

EXAMPLE 7

The seventh Example of the present invention is as follows. FIGS. 25A to25C are schematic views illustrating an embodiment of a μTAS(micro-total analysis system) sensor system using the three-dimensionalphotonic crystal 2207 of the present invention.

FIG. 25A is a schematic overhead image of the position of the μTASchannels and the photonic crystal laser. Channel 2202 is formed on achannel substrate 2201, in which a fluid 2203 containing detectedinformation flows. The structure of the channel 2202 may be mixing,reaction and other various structures used in μTAS in addition to branchand convergence shown in the figure. A photonic crystal laser sensor2204 is provided beneath the channel 2202 as represented in aperspective manner. FIG. 25B illustrates a laser sensor 2204 formed onthe upper surface of the three-dimensional photonic crystal layer 2207.

The photonic crystal laser sensor 2204 has a laser oscillation, whichvaries in an extremely sensitive way due to the concentration of thesubstance contained in the fluid flowing through the channel 2202, therefractive index, the temperature and the pressure of the fluid. Thecharacteristic thereof is to detect the oscillation of the laseraccording to the output of the laser beam. Accordingly, in this Example,as shown in FIG. 25C, the output of the laser beam 2208 is detected by alight-receiving layer 2209, which is the lowest layer, and theoscillation of each laser sensor 2204 is detected.

The oscillation of the laser sensor can be detected by a method otherthan detecting the output of the laser beam. In the case of exciting thelaser by injection of current, the oscillation of the laser can be alsomonitored by the variation of the injected current.

FIG. 25C is a schematic cross-section of the sensor system of thisExample. Channel 2202 is formed on a channel layer 2201 as describedabove, and a cover layer 2205 is formed thereon to close the channel2202 in the lamination direction. The other lamination direction isclosed by a thin film 2206, where a photonic crystal laser 2204 isformed in contact with the thin film 2206. The thickness of the thinfilm 2206 is set to be the optimal value to the level of the oscillationwavelength of the photonic crystal laser. The optimal value isdetermined so that the evanescent light from the resonator of thephotonic crystal laser senses the variation in channel 2202 and fluid2203 and so that the fluctuation of the threshold due to the loss in thelaser resonator is near the condition that enables oscillation.

When the thin film layer 2206 above the photonic crystal part is formedby epitaxial growth, a high performance element with excellent surfacesmoothness and low light scattering loss can be fabricated according tothe same technique as used in the method of fabricating thethree-dimensional photonic crystal of the present invention.

As described above, a μTAS sensor system can be formed by using thethree-dimensional photonic crystal and the method of fabricating thesame in accordance with the present invention. According to thisExample, such a μTAS system can be formed in a large area, and a largenumber of devices can be obtained from a wafer having a large area. As aresult, there is an advantage in that the fabrication can be conductedat a relatively low cost. In the case of forming a μTAS system usingsilicon or SiO₂, integrated formation is possible. Even in the casewhere each part is separately formed and jointed, the compatibility ofthe materials is extremely high.

EXAMPLE 8

The present invention is not limited to the Examples described above andthe flow of the sequence and other factors can be modified accordinglywithin the spirit of the present invention.

In particular, the present invention is not limited to materials such asSi, GaAs, Ge and GaP and can be similarly carried out in a combinationof a III-V compound semiconductor, such as AlGaAs, InGaAs, InAs,GaInNAs, InGaP and InP, a II-VI compound semiconductor, such as CdSe andCdS, and an epitaxially grown material that has a similar latticeconstant and linear expansion coefficient with a seed substratematerial.

The principle of the present invention is widely applicable and notlimited to those having an almost periodic refractive index distributionpattern, but can be applied to the fabrication of an optical element forthree-dimensional structural body having a non-periodic or randomrefractive index distribution pattern. In addition, the formation of therefractive index distribution pattern can be carried out after jointingthe layers, and the refractive index distribution pattern can also beformed even in the embodiments in which the refractive indexcontinuously changes as in the case of conducting etching on a crystallayer with constant change in the etching depth or the case of applyingdoping, such as ion injection, in which the concentration of impuritieschanges continuously, in addition to the embodiment of forming space orthe embodiment involving a non-continuous and relatively sharp change,as described in the above Examples.

This application claims priority from Japanese Patent Application Nos.2003-377638, filed Nov. 7, 2003, and 2004-317579, filed Nov. 1, 2004,which are hereby incorporated by reference herein.

1. A method for production of an optical element comprising the stepsof: (A) forming a first layer on the surface of a first substrate byepitaxial growth or micropores-making; (B) forming a porous layer on thesurface of a second substrate; (C) forming a second layer on the surfaceof the porous layer by epitaxial growth or micropores-making; (D)bonding the first layer and the second layer to each other; and (E)separating the second substrate from the second layer at the porouslayer.
 2. The method according to claim 1, further comprising step (F)of forming a refraction index distribution pattern on the first layerafter step (A).
 3. The method according to claim 1, further comprisingstep (G) of forming a refraction index distribution pattern on thesecond layer after step (C).
 4. The method according to claim 1, whereinsaid steps (B) to (E) are repeated with the outermost layer on the firstsubstrate as the first layer after step (E).
 5. The method according toclaim 4, wherein the refraction index distribution pattern formed on thesecond layer is a pattern having a periodic refraction indexdistribution formed in at least one direction as a result of repeatingsteps (B) to (E).
 6. The method according to claim 4, wherein the secondsubstrate in step (B) in a cycle is reused as the second substrate instep (B) in a following cycle.
 7. The method according to claim 1,wherein step (B) is a step of forming a porous layer comprised of twolayers different in porosity and the separating in step (E) is carriedout at the boundary of the two layers.
 8. The method according to claim4, further comprising a step of placing a light emitting element layerbetween layers.
 9. A method for production of an optical elementcomprising the steps of: (a) forming alternatingly porous layers andmicroporous layers on a first substrate; (b) forming a refraction indexdistribution pattern on the microporous layers collectively; (c) bondinga second substrate to the porous layer or microporous layer being theoutermost layer; (d) separating a pair of microporous layers spaced bythe porous layer at the porous layer; (e) shifting the separatedmicroporous layers from each other along an in-plane direction, andbonding the shifted microporous layers to each other; and (f) repeatingsaid steps (d) and (e) for each porous layer.
 10. An optical element inwhich layers having refraction index distribution patterns formedthereon are deposited to form a three-dimensional periodic distributionof refraction index, wherein a period of the refraction index in adirection of deposition is determined by the thickness of the pluralityof layers and a sequence of refraction index distribution patterns ofthe deposited layers.
 11. The optical element according to claim 10,wherein a light emitting layer is placed between the layers to form aphotonic crystal laser.
 12. The optical element according to claim 11,wherein a light emitting layer having in the direction of deposition aplurality of periodic refraction index distributions different to eachother and emitting a light of a wavelength corresponding to each periodis placed two-dimensionally along the layer.
 13. An optical devicecomprising: a light source layer emitting a light of a specificwavelength; and a photonic crystals being formed on the light sourcelayer and including a light emitting layer which receives the lightemitted from the light source layer and emits a light of a wavelengthdifferent from a wavelength of the received light, wherein the photoniccrystals are photonic crystals in which the wavelength of the lightemitted from the light source layer is out of a photonic band gap andthe wavelength of the light emitted from the light emitting layer iswithin the photonic band gap.
 14. The optical device according to claim13, wherein a plurality of photonic-crystal layers are stacked, and thewavelength of a light emitted from the light emitting layer in a lowerphotonic crystal layer is out of a photonic band gap of an upperphotonic crystal layer.